Oxygen adsorbed magnesium oxide

Magnesium oxide may be expected to function as an active catalyst for CO2 utilization since CO2 is easily chemisorbed on MgO [1-3]. Most works so far reported on this adsorption system were carried out above the room temperature, and recent works have demonstrated the occurrence of oxygen isotope exchanges between adsorbed species and surface 0 [4-6]. However, dynamic behaviors of adsorbed species, especially below the room temperature, are still open to question. 

Nanocrystalline MgO also has a large capacity for adsorbing acid gases such as S02, C02, HC1, HBr, and S03 at near stoichiometric proportions. Work by Klabunde has demonstrated that nanocrystalline magnesium oxide has an enhanced surface reactivity over that anticipated from surface area alone. Nanocrystalline MgO chemisorbed 6 molecules of S02 per nm2, whereas larger microcrystalline MgO only chemisorbed 1.8 molecules per square nanometer (Stark and Klabunde, 1996). The proposed mechanism for the difference in adsorption capacities is due to a monodentate-type adsorption mechanism of S02 via the sulfur atom in the instance of nanocrystalline MgO, whereas microcrystalline MgO favors a bidentate adsorption mode through sulfur and an oxygen atom. 

In Nature (on membranes of chloroplasts) water is oxidized by the four-electron mechanism and the life-time of chlorophyll can be as long as one day. In a model octane/water system, oxygen evolution occurs also during several hours. This is indirect evidence in favor of a many-electron reaction. Therefore we shall consider a four-electron mechanism of water oxidation sensitized by chlorophyll adsorbed at the oil/water interface proposed by Volkov [81]. It was shown above that the interface is the most likely site for the water photooxidation reaction. Thus it is assumed that water can be oxidized by a reaction complex adsorbed at the interface that consists of a hydrated oligometer of chlorophyll, a hydrophilic electron acceptor and hydrophobic proton acceptor [81,82,86]. The water in the reaction complex is linked coordinatively with the magnesium of one of the chlorophyll molecules, by hydrogen bonds with the carbonyl group of another chlorophyll molecule, and with the phenol anion also... 

While a surface structure with (2 x 2) periodicity has been observed, Ciston and coworkers concluded that the observed diffraction intensities do not match with an octapolar structure [20]. Instead, the diffraction data indicates a (2 x 2)-a type structure. In this structure, the atoms in the terminal layer can occupy any or all of three different possible sites (Fig. 12) and the different occupations are virtually indistinguishable crystallographically. Assuming that Mg and O can only have oxidation states of 2 and no adsorbates are present, two of the three sites must be occupied in order to maintain valence neutrality. Like the octapolar structures, the (2 x 2)-a- can have either a magnesium bulk termination with the surface sites filled by oxygen ((2 x 2)-a-0), or an oxygen bulk terminatimi with the surface sites filled by magnesium ((2 x 2)-a-Mg). The (2 x 2)-a-Mg structure considered by Ciston has all three sites occupied by Mg atoms, would not be valence neutral, and does not appear to exist. A (2 x 2)-a-Mg structure with two sites occupied would be valence neutral, but the terminal Mg atoms would not satisfy the valence vector sum rule. 

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